Grounding without Ground

My system use only Line and neutral connection form mains ( No ground) , Inside this system it converted to
5V using SMPS. My enclosure is metal , I need to Ground this enclosure , Since I do not have ground wire coming in my system
,So i am going to connected my Circuit return ground (5V return ground) to connected to the enclosure,
How effective the shield will be on enclosure by doing this.